High-intensity discharge (HID) lamps containing ceramic arc tubes are well known. Such lamps include high pressure sodium lamps and metal halide lamps which contain translucent polycrystalline (PCA) arc tubes. In the case of metal halide lamps, the arc tubes have opposed capillary tubes extending outwardly from an axially symmetric body. Each capillary tube contains an electrode assembly which provides the electrical energy needed to strike the arc discharge inside the discharge vessel. The end region of each capillary tube is sealed hermetically to the electrode assembly with a frit material. Examples of such arc tubes are described in U.S. Pat. Nos. 5,973,453 and 5,424,609, and European Patent Nos. 0 971 043 A2 and 0 954 007.
One state-of-the-art method uses radio frequency (RF) heating to form the hermetic seals in the capillary tubes. U.S. Patent Publication No. 2002/0117965, which is incorporated herein by reference, describes a method for sealing a ceramic arc tube by RF induction heating. The RF sealing apparatus comprises a resealable pressure chamber with an RF induction heater mounted at one end. The RF induction heater is comprised of an RF power supply, an RF induction coil located external to the pressure chamber, and an RF susceptor located within the chamber. The end of the arc tube to be sealed is held within the RF susceptor, preferably a hollow graphite cylinder. During sealing, the RF susceptor absorbs energy from the RF induction coil causing the susceptor to heat up. The thermal radiation emitted by the hot susceptor in turn causes a ring of frit material mounted on the end of the capillary to melt and the molten frit flows into the open end of the capillary tube and down along the electrode assembly. When the RF power is removed, the frit solidifies forming a hermetic seal.
While this method is effective, when the susceptor is part of the apparatus, a series of susceptors must be designed, maintained, and installed to match the variety of arc tube sizes to be sealed. Graphite is often used as a susceptor material because it is machinable, electrically conductive while highly resistive, and can withstand high temperatures (˜3000° C.) in inert atmospheres. However, graphite susceptors have a limited lifetime, are somewhat fragile, and their electrical properties can vary depending on the manufacturing method.